Plans for televised election debates have descended into chaos after David Cameron declared he had struck a deal to appear in a single face-off.

The Prime Minister said the broadcasters had made him a "formal offer" that would see one debate held featuring seven party leaders on April 2, but two others - including a head-to-head with Ed Miliband - dropped.

The announcement appeared to catch other political parties and the broadcasters by surprise. In a joint statement released hours later, the BBC, Sky, ITV and Channel welcomed Mr Cameron's agreement to a "first" debate - but said talks were still ongoing about formats for other election programmes.

Labour made clear it was still working on the basis that three encounters were planned, with Ed Miliband accusing the Tory leader of " going to any length to avoid a head-to-head debate".

However, the Liberal Democrats hailed the "latest proposals" from the broadcasters - which would apparently see the main party leaders interviewed separately on programmes alongside the single seven-way debate - as progress towards a "sensible solution".

The broadcasters had previously mooted three debates, with the first two on April 2 and 16 featuring Mr Cameron, Mr Miliband, Liberal Democrat leader Mr Clegg, Ukip's Nigel Farage, Green leader Natalie Bennett, Nicola Sturgeon of the Scottish National Party and Plaid Cymru's Leanne Wood, followed by the head-to-head encounter on April 30 between the two men vying to be prime minister.

They had threatened to "empty chair" the Prime Minister in any debate he failed to attend.

It is understood that a small group of TV executives have been in talks with the Conservatives since Mr Cameron insisted he would not back down on his "final offer" of a single debate to take place before the election campaign begins on March 30, but it remains unclear whether other parties were involved.

Tory sources said the package of programmes agreed by the PM includes an interview by Jeremy Paxman featuring questions from a studio audience, with Mr Miliband attending a similar event.

Then, on April 2, there would be a seven-way televised debate, including Mr Cameron and other party leaders.

A further "challengers debate" would feature the leaders of the smaller parties.

On April 30, a week before polling day, Mr Cameron, Mr Miliband and Nick Clegg would take part in a rotating question time event, following one another into a television studio but not going head-to-head.

Mr Cameron said: "There was a formal offer of a set of television programmes including a televised debate, put together by the broadcasters led by the BBC.

"As Prime Minister, I accepted that deal in full."

He added: "This was an offer put together by the broadcasters, accepted by me on Saturday. The other parties have now got to stop running away from this and agree to the debate that the broadcasters have now suggested."

Asked if he was getting out of a head-to-head debate with Mr Miliband he said: "What I'm getting into is a set of television programmes and a televised debate."

But Mr Miliband hit back: "I'm fighting for the debate between me and David Cameron which the British people want.

"We now have the tawdry spectacle of the Prime Minister going to any length to avoid that debate.

"I say to David Cameron, even at this late stage: agree to this debate which the British people deserve."

The joint statement issued by the broadcasters said: "In recent days the broadcasters have had detailed discussions with a number of the parties with the aim of delivering impartial debates and other programmes in the run up to the general election.

"We welcome David Cameron's willingness to participate in the first TV debate on April 2.

"The broadcasters are continuing to work with all the political parties on programmes scheduled for March 26, April 16 and April 30."

A Labour spokesman said: "Based on the broadcasters' proposals, we have accepted and plan to attend all three debates on April 2, 16 and 30."

A Lib Dem spokesman said: "It's good news that we are finally making progress towards a sensible solution on the TV debates.

"The latest proposals from the broadcasters are welcomed by the Liberal Democrats and Nick Clegg will take part in the events that he's been invited to."

Ms Sturgeon said: "While it is welcome that David Cameron has accepted his position was indefensible and agreed to debate during the campaign period - abandoning his arrogant 'final offer' - he should sign up to the full programme of debates that is on the table.

"I will debate David Cameron any time, anywhere, and on any number of occasions - but a Tory Prime Minister simply cannot be allowed to dictate terms to everyone else."

A Ukip spokesman said: "We agreed to the three debates the broadcasters proposed and will be turning up to the two we were invited to."

A tweet on the BBC press office Twitter feed suggested the PM had been "sounded out" on the latest debate arrangements.

"Good news that David Cameron's said for the first time he's willing to do the first debate on 2 April - even if it's on ITV," it said. "Of course, debates still work in progress. B'casters sounded out Cons on proposals, then Lab, Lib Dem & other parties. Discussions continue."